The three main states of matter are solid, liquid, and gas. At room temperature, most substances are in the liquid or gas state. For example, water is a liquid at room temperature, while air is a gas. Some substances, such as iron, are in the solid state at room temperature. The state of matter of a substance depends on its temperature and pressure. When the temperature of a substance is increased, it will eventually change from a solid to a liquid to a gas. When the pressure of a substance is increased, it will eventually change from a gas to a liquid to a solid.